Generate the dev guide `mdbook` pages of available teams

[tgross35]

From discussion at https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/I'd.20like.20to.20contribute.20more.20.28to.20Apple-specific.20stuff.29/near/434259224, there isn't an obvious place to see which notification groups exist. https://rustc-dev-guide.rust-lang.org/notification-groups/about.html exists in the dev guide but isn't complete.

Could this repo just generate the dev guide pages?

[keys-i]

are the notification groups in a toml?

[keys-i]

can we add them in a toml to make this easier